RELATIVE WEIGHT AND VOLUME OF COMPONENT PARTS OF FETAL BRAIN. 59 



cent in the new-born. At 11 weeks the archipallium could be subdivided into the 

 olfactory bulb, which weighed 0.3 per cent, and the paraterminal body, fornix, and 

 hippocampus, which together weighed 3.22 per cent. Both of these units show a 

 consistent shrinkage in relative weight until birth, when the former weighed 0.2 

 per cent and the latter 0.83 per cent. 



The diencephalon could also be isolated in all cases, and in the 4.3 mm. embryo 

 had a weight-value of 24.3 per cent, or more than three times that of the telenceph- 

 alon. From this high initial percentage the curve declined gradually to 2.63 per 

 cent at term, notwithstanding the considerable increase in the size of the thalami. 



The mesencephalon, weighing in the youngest specimen 14.3 per cent, drops 

 steadily to 0.64 per cent at term, declining from a structure of considerable size 

 to a mere slender, connecting stem, which attains its chief differentiation through 

 the development of secondary centers connected with the fiber systems passing 

 through it. 



The total rhombencephalon starts with a maximum weight of 54.4 per cent of 

 the whole in the 4.3 mm. specimen, from which point its curve sinks rapidly until 

 at 156 mm. it reaches a minimum of 4.97 per cent, then rises gradually to 5.26 per 

 cent at 230 mm., and 6.98 per cent at term. This unusual curve is explained when 

 we study the growth-rate of the two component units of the rhombencephalon, 

 which behave in a widely different manner. The curve for the cerebellum begins in 

 the 16 mm. embryo at 7.56 per cent, ascends to 9.74 per cent at 8 weeks, its maximum 

 point during prenatal growth. From this level it falls to 2.74 per cent at 13> weeks, 

 remaining there until the end of the nineteenth week when it rises rapidly, reaching 

 6 per cent at term, its actual weight being 30.7 grams. This reversal of the usually 

 observed conditions shows the increasing importance of the cerebellum at these 

 stages of development, its individual growth-rate becoming sufficiently marked to 

 maintain an unvarying level during six weeks of changing values in other parts. It 

 then gains a new impetus which is sustained throughout of the remainder gestation. 

 The medulla-pons in the 16 mm. embryo comprises 38.11 per cent of the weight of 

 the entire encephalon. The curve falls rapidly to 3.73 per cent at 13>^ weeks, then 

 more gradually to its minimum of 0.91 per cent at term, the general weight-curve 

 for this part being comparable only to that of the diencephalon, though the relative 

 loss is much greater in the medulla-pons. It would seem probable that the early 

 large size of the medulla-pons is due to its association with the development of the 

 cranial nerves which are relatively very large at this time. 



